Add a POST form to the landing page

web/landing_page.css

@@ -5,9 +5,13 @@ body {
 header {
   background-color: {{.HeaderColor}};
   color: #fff;
-  font-size: 2rem;
+  font-size: 1rem;
   padding: 1rem;
 }
 main {
   padding: 1rem;
 }
+label {
+  display: inline-block;
+  width: {{.Form.Width}}em;
+}

web/landing_page.go

@@ -31,10 +31,27 @@ type LandingConfig struct {
 	CSS         string         // CSS style tag for the landing page.
 	Name        string         // The name of the exporter, generally suffixed by _exporter.
 	Description string         // A short description about the exporter.
+	Form        LandingForm    // A POST form.
 	Links       []LandingLinks // Links displayed on the landing page.
 	Version     string         // The version displayed.
 }
 
+// LandingForm provides a configuration struct for creating a POST form on the landing page.
+type LandingForm struct {
+	Action string
+	Inputs []LandingFormInput
+	Width  float64
+}
+
+// LandingFormInput represents a single form input field.
+type LandingFormInput struct {
+	Label       string
+	Type        string
+	Name        string
+	Placeholder string
+	Value       string
+}
+
 type LandingLinks struct {
 	Address     string // The URL the link points to.
 	Text        string // The text of the link.
@@ -54,6 +71,15 @@ var (
 
 func NewLandingPage(c LandingConfig) (*LandingPageHandler, error) {
 	var buf bytes.Buffer
+
+	length := 0
+	for _, input := range c.Form.Inputs {
+		inputLength := len(input.Label)
+		if inputLength > length {
+			length = inputLength
+		}
+	}
+	c.Form.Width = (float64(length) + 1) / 2
 	if c.CSS == "" {
 		if c.HeaderColor == "" {
 			// Default to Prometheus orange.

web/landing_page.html

@@ -19,6 +19,16 @@ <h1>{{.Name}}</h1>
           {{ end }}
         </ul>
       </div>
+      {{ if .Form.Action }}
+      <div>
+      <form action="{{ .Form.Action}}">
+      {{ range .Form.Inputs }}
+      <label>{{ .Label }}:</label>&nbsp;<input type="{{ .Type }}" name="{{ .Name }}" placeholder=" .Placeholder }}" value="{{ .Value }}"><br>
+      {{ end }}
+      <input type="submit" value="Submit">
+      </form>
+      </div>
+      {{ end }}
     </main>
   </body>
 </html>